The incident happened Monday afternoon on Southside Avenue near Guy Lombardo Avenue in Freeport.
Police say the robber flashed a knife at the ninth grader, then ripped a gold necklace off her neck.
The teen was not hurt.
The victim further described the subject as being a dark-skinned black male, about 20 to 25 years old, 5-foot-4 with a medium build. He was wearing dark-colored shorts, a dark tank top and a white towel that was wrapped around his head.
Detectives request anyone with information regarding this Robbery to call Crime Stoppers at 1-800-244-TIPS. All callers will remain anonymous.